It's pretty much what has been said here. Various things can change the color. A bright green is generally from an artificial coloring, or the stool passing through the colon faster than normal. Cereals with bright colors like fruit loops or others are a common reason for this. It's also common to get a black stool from pepto, and black licorice will give you a very dark stool...somewhere in between a dark green and black.
Also, to the people who say why do you look at your stool, it's a very good thing to do. Changes in stool color or consistency can be a early warning sign for various colonic related disorders. Most doctors will tell you to pay attention to the shape too.
Black, tarry stool for a long period of time is probably the worst one, and can be an indication of fairly heavy bleeding further up in the colon caused by colon cancer or other disorders. It's not so big of a deal for younger people, but the earlier you are diagnosed with colon cancer, your survival rate goes up dramatically. Blood in stool from higher up in the colon will usually be black, while blood from further down will be red. The black blood, "occult blood" I beleive it's called, is the worst while the red is fairly normal from time to time.